Hand-in-hand with the vigorous growth of the polyurethane production industry, there has been a concomitant increase in the problem of removing and re-using polyurethane waste or reject goods. Although a market has been found for flakes of flexible polyurethane foam by glueing the flakes together to form a composite material, only a limited quantity of flexible foam may be utilized in this way. There is no similar possibility of using waste from semi-rigid or rigid polyurethane foams or from elastomer granulates. Large quantities of polyurethane waste and reject goods from the production of rigid and flexible foams and the production of elastomers must therefore be stored in depots or destroyed in refuse incineration plants. This gives rise to considerable ecological, technical and economic problems because of the low specific gravity and hence large bulk of the waste and reject goods.
There is therefore considerable interest both on ecological and on economic grounds for industrially utilizing the ever-increasing quantities of polyurethane waste.
Processes have been disclosed for this purpose in German Offenlegungsschriften Nos. 2,362,919; 2,362,920 and 2,362,921, according to which polyurethane foam waste is hydrolyzed using steam at high temperatures. These processes require high temperatures and pressures (for example, 240.degree. C. and 40 atmospheres) so that the dissociation of polyurethane waste may only be achieved at great cost and considerable outlay LeA 17,948 for equipment. Moreover, the reaction products are obtained as mixtures with water so that they must be isolated by specialized processes before they may be re-used.
German Offenlegungsschrift No. 2,238,109 describes how to degrade polyurethane foam waste into refoamable polyols by heating to a temperature of from 175 to 250.degree. C. in high-boiling dihydroxy compounds, preferably diethylene glycol and preferably in the presence of approximately 10% of a diethanolamine. Under these conditions, a transurethanization process takes place by which the polyurethane is converted into short chain polyols which may only be used advantageously for the production of rigid polyurethane foams. According to German Offenlegungsschrift No. 2,238,109, this process is preferably only used for breaking down rigid polyurethane foams. The main disadvantage of this process is that decomposition proceeds so slowly that addition of the foam waste takes several hours and the reaction mixture must still be stirred for some time after all the foam has been added. The process therefore consumes a great deal of energy and is difficult to operate on a continuous basis.
According to an earlier proposal by the present applicants (U.S. application Ser. No. 723,872, filed Sept. 16, 1976), now U.S. Pat. No. 4,115,298 polyurethane waste is dissolved in lactam melts or in associates of lactams and adduct-formers having at least two Zerewitinoff-active hydrogen atoms at elevated temperatures. The types of polyurethane bonds which cause chain-lengthening and chain-branching are thereby opened. This process leads, within very short reaction times, to activated polyhydroxyl compounds which may be used again as starting components for the production of polyurethane resins, such as flexible or rigid polyurethane foams or polyurethane elastomers. If, however, such activated polyhydroxyl compounds are used as the only polyol component for the production of rigid polyurethane foams, only very brittle, crumbly foams of inferior quality are obtained, which, for many fields of application, do not meet the market requirements. To improve the properties of polyurethane resins produced from the activated polyhydroxyl compounds described above, it is therefore recommended to mix the resultant polyhydroxyl compounds with other low molecular weight or higher molecular weight polyhydroxyl compounds, such as polyethers and/or polyesters. However, since the activated polyhydroxyl compounds obtained according to U.S. Ser. No. 723,872 are highly viscous masses, particularly those which have been obtained from rigid polyurethane foams, such mixing is not without its technical difficulties. Many of the commercial polyhydroxyl compounds such as those conventionally used as starting components for the production of high quality rigid polyurethane foams are quite impossible to mix homogeneously with the activated polyhydroxyl compounds. Inhomogeneous polyol mixtures, however, are for the most part unusable for processing in present-day foaming machines and for obtaining high quality rigid polyurethane foams.